The present invention relates to a method of strengthening a brittle oxide substrate and also relates to aqueous solutions containing silane-based compositions and polymerized cross-linked siloxane coated brittle oxide substrates. More particularly, the present invention relates to a method of strengthening or restoring strength to a glass container and the resulting polymerized cross-linked siloxane coated glass container.
Brittle materials, such as glass substrates, generally exhibit some mechanical properties, such as, e.g., tensile strength, which are substantially lower than predicted. This manifestation can arise as the result of such factors as imperfections in the structure of a test specimen, or small amounts of impurities in either the body or the surface of an article made of that material. Progressive zone melting to reform the crystalline structure and floating impurities out of the melted brittle material have been used in the past for brittle metals in an attempt to improve the mechanical properties of the brittle metals. Also, with regard to non-metal brittle materials, multi-layer structures made of the brittle material have been used to improve mechanical properties. In addition, surface treatments of the brittle material have been used to protect the surface from abrasion and to provide a small measure of support to brittle articles.
Glass is intrinsically one of the strongest materials known to man. Theoretically, standard silicate glasses should be able to support stresses as high as 14 to 20 gigapascals (2 to 3 million pounds per square inch (psi)). In practice, however, the strengths typically obtained are on the order of 70 megapascals (MPa), about 10,000 psi.
The explanation of the discrepancy between predicted and measured values is the existence of surface flaws or cracks. These flaws essentially fracture the siloxane network (Si--O--Si), which is the backbone of the glass. This damage to the glass acts to concentrate any applied force to the point of causing catastrophic failure of the glass article, typically at much lower stresses than otherwise expected. While described here for glass, this same theory can be applied to any brittle material not demonstrating significant plastic deformation prior to failure.
In the case of a glass container, for example, the surface flaws or defects can originate from many sources, ranging from unmelted batch materials to scratches produced by sliding across hard surfaces, including other glass articles. In a typical container-manufacturing facility for example, the glass articles can be heavily damaged by handling from the moment they are formed. Contact with particulates and moisture in the air, other bottles, guiderails and other handling equipment, and the conveyor on which they are transported, can lead to large decreases in the strength of the container due to the flaws produced.
Researchers have long sought a means to alleviate the problems with glass strength. Many modifications to the forming and handling process have led to unsatisfactory increases in the strength because these modifications in handling still leave flaws in the surface. For this reason, it has been a goal of researchers to reduce the effect of flaws after they are inevitably formed on the object.
Some approaches to improving the strength of glass include Aratani et al., U.S. Pat. No. 4,859,636, wherein metal ions in the glass are exchanged with ions of a larger radius to develop a surface compressive stress. Poole et al., U.S. Pat. No. 3,743,491, also relates to a surface compressive stress, but provides a polymer overcoat to protect the surface from further abrasion. Hashimoto et al., U.S. Pat. No. 4,891,241, relates to treating the surface of the glass with a silane coupling agent followed by a polymer coating containing acryloyl and/or methacryloyl groups, followed by irradiation or thermal treatment to polymerize the molecules containing those groups. The '241 patent further shows that silanes alone do not strengthen substrates and that acrylates are necessary for any strengthening.
While the patents described above each provide some improvement to the strength of the glass so treated, they are not without shortcomings. Some of these treatments require longer times than available during manufacturing, necessitating off-line processing. There are also concerns related to worker safety and health. In particular, the use and handling of organic solvents, as well as the acrylate and methacrylate compounds, are a safety and health concern to the manufacturer.
Therefore, there is an unmet need for a method of strengthening a brittle oxide substrate which addresses the above concerns as well as provides acceptable increases in strength to the brittle oxide substrate. There is also a need for a coated brittle oxide substrate which has a substantially improved strength when compared to a brittle oxide substrate without any coating.
Further, there is a need for a method of strengthening a brittle oxide substrate which will also provide acceptable labelability and/or humidity resistance.
In addition, there is a need for a polymerized cross-linked siloxane coated brittle oxide substrate wherein the cured coating is transparent.
Additional objects and advantages of the present invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the present invention. The objects and advantages of the present invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.